British Patent Specification No. 1577322 discloses a method of reducing the amplitude of sound variations received at a selected location from a source of recurring noise which employs a waveform generator producing a synthesised waveform capable of being used to generate a cancelling sound in the location for the noise entering that location and uses a triggering signal, derived from the source, to accurately relate the generation of the cancelling sound to the noise it is wished to cancel. This prior specification (the entire disclosure of which is herein incorporated by reference) forms the basis for the present invention.
It is also known from Wheeler and Halliday's published representation (entitled "An active noise reduction system for aircrew helmets--flight trials in strike aircraft") presented at Birkbeck College, London, on Feb. 16, 1981, to mount an active noise reduction system in a passive ear defender to cancel the acoustic noise field detected inside an ear defender. However this known arrangement is incapable of distinguishing between components of the noise field and is thus of limited use and is not effective at all for improving the audibility of random signals appearing in a high background of repetitive noise signals.